This invention relates to a process for the removal of hydrogen sulphide from gaseous mixtures by scrubbing with an aqueous alkaline scrubbing solution containing an oxidizing agent. The hydrogen sulphide is absorbed from the scrubbing solution in a scrubbing column and oxidized principally to sulphur, as well as a minor amount of sulfate (SO.sub.4.sup.--) and thiosulfate (S.sub.2 O.sub.3.sup.--). The scrubbing solution containing resultant reduced oxidizing agent is preferably subsequently regenerated for reuse in a downstream oxidizer by exposure to an oxygen-containing gas.
A process of this type is described, for example, in British Patent Specification No. 1,488,659. According to this process, the gas to be cleaned is scrubbed with an aqueous alkaline solution, such as a sodium carbonate solution having a pH between 8 and 9, this solution containing a soluble salt, such as the sodium salt of anthraquinonedisulphonic acid, and a metal vanadate, the vanadium being in the five-valent oxidation state. The absorbed hydrogen sulphide dissociates in the solution with the formation of HS.sup.- ions which reduce the vanadium to the four-valent oxidation state, elementary sulphur being liberated. The reduced vanadium can be re-oxidized by an oxidation reaction with an oxygen-containing gas in the presence of the anthraquinonedisulphonic acid.
It has been found, however, that part of the hydrogen sulphide is oxidized to soluble sodium sulphate and sodium thiosulphate which gradually build up in concentration in the scrubbing solution and must be removed. Whereas the removal of sodium sulphate can be accomplished, for example, by simple crystallization, this is not possible for sodium thiosulphate because of its high solubility. Furthermore, since sodium thiosulphate is a substance which is ecologically deleterious because of its high biological oxygen demand, the scrubbing solution containing the buildup of S.sub.2 O.sub.3.sup.-- cannot simply be discarded. Thus, according to the British Patent Specification No. 1,488,659, it is proposed to add to the scrubbing solution a water-soluble aromatic polyhydroxy compound, such as dihydroxybenzene, dihydroxynephthalene or dihydroxyanthracene, which prevents the formation of sodium thiosulphate. This procedure, though useful, has the disadvantage of using costly additives, the recovery of which involves further problems.